Articulated flapping wings
Description of flapping wing constructions
which have been developed together with the EV-models
1. Aeroelastic controlled articulated flapping wing
The design layout demands on a technical flapping wing-among other things-results from the theoretical distribution of lift based on Robert T. Jones (USA 1950 and 1980) and the therefore necessary angle of incidence along the wing semi-span.
Here, for example, the relevant, extensively optimised functional distributions
for a gently inclined flight of a rectangular flapping wing with the all-round
airfoil CLARK-Y are shown (principles please see Handbook).
In this case, the angle of incidence at the wing root remains constant during
the whole flapping period. The distributions of the downwash angle along the
span are straight-lined in all three cases (please look at the diagram
distributions on the wing).
In the flapping wing design the momenta of inertia round the stroke axis and the rotating axis of the flapping wing are also important.
This isasic spar framework of a profiled flapping wing with an articulation for an additional flap motion of the hand wing spar. This is pulled down by a spring (spring device here not shown).
- While gliding flight with its medium lift force, the hand wing takes over the stretched center position.
- On downstroke in the hand wing area the lift forces distinct increase and the hand wing strokes up against the spring force.
- If on upstroke the lift forces decrease the spring force pulls down the hand wing.
The small, on aerodynamic forces dependent and thereby aeroelastic stroke motion of the hand wing will be used by levers (brown for arm wing, green for hand wing) to control the twisting along the whole wing. In this way an articulated flapping wing with aeroelastic controlled twisting has been developed.
With the exception of the wing root rib all ribs are put freely rotatable on
the spars. For the covering of the flapping wing the highly elastic polyurethane
Platilon U 04 is planed (please look at related
The mode of operation of this articulated flapping wing, with its wing twisting by wing bending against the stroke direction, resembles a little to that of a bird's wing. But according to studies by Karl Herzog at birds the motion of the hand wing happens in another way. For them, the rotary motion of the hand wing is mechanically coupled with the forward and backward motion of the wing tip when spreading and pulling up the wings. This coupling is tighly in the stretched wing position and with increasing back-swept wing tips only loosely (please take a look at the book
Wrist joint with the torsion lever of the arm wing
and the wire rope to lock the wing twisting in gliding
Function of the wrist joint of the wing
The yellow lever shows the rotation of the hand wing.
The principle of this aeroelastic controlled articulated flapping wing I have developed in connection with the flapping wing model EV6 (1983). It has been continually improved and applied in the subsequent models.
Ornithopter in flight, during wing upstroke
2. With increasing wing twisting
near the wing tip
This is the functional model of a
stroke amplitude expanding wing spar
in short form called
shift spar. This mechanism can be used at main
and at auxiliary spars of flapping wings.
If the shift spar is used as a rear auxiliary spar, it can be used to increase the wing twist in the range of the wing tip.
Shift linkage dismounted
Shift linkage mounted
The left hinge here provide as basis. At a stroke moving of the middle shift linkage - here downward - the outer right spar section implements expanded stroke amplitude.
for a shift spar in a coaxil tube-in-tube construction
GRP joints of the shift spar
with different center distances for transmitting the flapping motion (1:1.5).
Adjacent one can see the spar framework of a profiled flapping wing for active wing twisting at the arm wing section by main spar rotation.
There is a
stroke amplitude expanding auxiliary wing
spar is used. In doing so the twisting at the hand wing section is increased.
The spar shift linkage is hereby mounted coaxially.
The rib at the wrist is fixed firmly to the main spar and the auxiliary spar hinges fixed firmly to the wing root rib. All the other ribs are stuck on the spars rotating freely. The covering is done with an elastic foil.
For a passive or aeroelastic twisting the rib at wrist should to be pivoted on the main spar.
3. With adjustable twisting moment
By splitting the flapping and twisting tasks of a flapping wing on a main and an auxiliary spar its twisting moment can be designed adjustable.
Adjacent, the framework configuration of an aeroelastic twistable flapping wing with an adjustable twisting moment becomes obvious. The adjustment is affected by the torsion lever at the wing root.
The rest of the other ribs not shown here are fixed to the spars rotating freely. The covering is done with an elastic foil or according to the Shearflex principle. Also shell and foam wings can be designed adjustable this way.
If you apply this system on a non-flapping aerofoil, a propeller blade or a wind turbine blade, their twisting can be controlled by the auxiliary spar.
Generally, the different types of wing systems can be combined together in many ways.
In the adjacent picture for example, the auxiliary spar of the arm wing with its torsional elastic force is used as a spring device for the small flap moving of the spar of the hand wing of an aeroelastic controlled articulated flapping wing.
The arm wing torsion linkage (AT) and the arm wing auxiliary spar (AHi) are hereby fixed firmly together. This way, the auxiliary spar-torsional moment will be transformed into a torque of the torsion linkage (AT). This is pivoted at the front and presses down the spar of the hand wing in the indicated rotary direction. The pressure will be adjusted with the inlying lever (InH) at the wing root.
The profiled arm part of this articulated flapping wing can also be combined with a membrane hand wing. This is possible a useful design for medial climbing and gliding flights.
4. Aeroelastic controlled articulated flapping
wing with adjustable twisting moment and
expanding wing twisting at the wing tip
Wing framework of the model EV8
(2004), designed as an aeroelastic controlled articulated flapping wing, combined
stroke amplitude expanding auxiliary spar of the hand wing at
the wing tip section.
- The twisting elasticity of this flapping wing can be adjusted by the torsion of the auxiliary spar of the arm wing at the wing root.
- The downstroke twisting is slowed down by a dashpot.
- The twisting when gliding can be fixed by a radio-controlled servo.
The downstroke twisting here corresponds with the theoretical guidelines almost to the wing tip (please also look at the picture which shows the wing structure of the EV8 in the upstroke twisting). But still disturbing is the high mass moment of inertia of this flapping wing round its axis of flapping and twisting.
- Maximum intended wing twisting (click on picture)
In flight practice the articulated flapping wing has a big advantage. The bending of the hand wing in comparison to the arm wing depends on lift of the hand wing. At the same time it determines the distribution of the angle of incidence along the wing span. If the amplitude of the bending is estimable on flight pictures, the lift forces of the hand wing in comparison to the gliding flight can be estimated. Furthermore, the distribution of the angle of incidence in the moment of the picture was taken can easily be suggested (please look at EV6 and EV7). With these tow information's selective adjustments of the twisting moment of the flapping wing, the driving power and the cycle time ratio are possible. Especially flight pictures taken approximately in the middle of up- and downstroke are informative.
In this paper you can find some more details about
Articulated flapping wings (in German, PDF 1.3MB).
Informations and suggestions for a further development you will also find in the article Lift during wing upstroke (version 7.0, PDF 0.7 MB).
5. Covering of flapping wings
As well as for the other EV-models a 0.050 mm thick elastic polyurethane-foil was used as cover for the aforesaid wing.
Double-sided adhesive tape was used adhere the foil to the wing framework (here still covered with release paper).
Version of a trailing edge for flapping wings
composed of a fishing line at the end of the airfoil wrapped with an adhesive tape.
Here the description of the manufacturing processes of the
Covering of a flapping wing with an elastic film
(version 1.1, in German, PDF 360 KB)